This invention relates to an improved filter for use in air conditioners and other such devices through which air is passed and filtered. More especially the present invention relates to a sleeve filter having a plurality of filter layers on the upstream side of a supporting wire frame and a thin fibrous surfacing mat on the downstream side of the frame. The layers on the upstream side of the frame form a multi-stage fluid filter having a thickness substantially greater than the thickness of the surfacing mat on the downstream side. The upstream filter layers and the surfacing mat are tensioned across the frame to prevent bulging. The filter is self-gasketing to prevent by-pass and blow-out.
Filters for use in air conditioning and heating devices are known and include filters having a metal or cardboard frame extending about the edges of the filter and overlapping both the front and back panels of the filter. Some of these filters also have expanded metal grids or similar structures (such as bottle cap scrap) on one or both surfaces to support the filter. In some prior art filters, coarse screens have been embedded in the fibrous material for supporting the filter.
However, in all of these filters the support material considerably reduces the effective area of the filter, even up to 20% of the filtering area. Furthermore, when the edge type supported filter is used, particularly in devices where filters are abutted edge to edge, there results a gap or a space through which unfiltered air readily passes.
In U.S. Pat. Nos. 3,388,535 and 3,388,536, both to Nash, there are described self-supported filters each having two layers of fibrous filter material. Each patent describes a self-supporting structure as containing a self-supporting frame and centrally located supporting elements to prevent buckling between the two filter elements. The frame includes two portions, the first of which portions extends adjacent the entire periphery of both layers. The second portion is centrally disposed between the two filter elements for purposes of preventing buckling. In both patents the filter layers contain thermoplastic materials which are secured relative to the support frame elements by heat sealing the filter layers to each other.
Attempts have been made to eliminate the problems inherent in prior art filters. One answer to such device is described in U.S. Pat. No. 3,877,909 and includes a composite frameless filter having at least two fluid permeable filter members of substantially the same peripheral shape. A thin wire-like frame member is positioned between the filter members and is spaced inwardly from the periphery thereof. The first filter member constituting the upstream filter layer is relatively thick and comprised of a low density, medium to long filament puffed glass fiber layer while the second layer is positioned on the downstream side and is a thin, relatively high density non-woven fiber layer. Anti-bulge means are provided in this internally supported composite frameless filter which operates together with localized compression means centrally positioned on both sides of the composite filter to prevent the filter from bulging. Each of the filter members is adhesively secured to the frame member and is uncompacted throughout each of their entire thickness except in the area of the anti-bulge means.
While this filter works well in air-conditioning and heating devices, it has become desirable to provide an alternate means for solving the problems inherent in prior art filters and at the same time provide a more economic filter while retaining the advantages of the filter described in U.S. Pat. No. 3,877,909.